Digital temperature sensors are well known. Digital temperature sensors are typically used to generate a temperature reference for the circuits on the chip or other integrated chips.
Digital temperature sensors, such as voltage reference circuits, generate a temperature output signal by normalizing a thermometer voltage V.sub.TEMP, which is temperature dependent, to a reference voltage V.sub.REF. The analog to digital converter of the digital temperature sensor measures the temperature dependent voltage ratiometrically relative to the reference voltage. Thus, the output temperature signal of a digital temperature sensor can be expressed as follows: ##EQU1##
where F(T) is the thermometer transfer function, V.sub.TEMP is the temperature dependent thermometer voltage, and V.sub.REF is the reference voltage.
The reference voltage V.sub.REF is typically generated by a bandgap reference voltage circuit. As is well known in the art, a bandgap reference voltage V.sub.REF has two component voltages: a C*.DELTA.V.sub.BE (T) term, which is the scaled difference between two base emitter voltages and is proportional to absolute temperature (PTAT), and a V.sub.BE (T) term, which is one base emitter voltage. When these two component voltages, which have opposite and nearly equal temperature coefficients, are summed together correctly, as follows: ##EQU2##
the resulting reference voltage V.sub.REF is nearly temperature independent. Unfortunately, there are second order errors intrinsic to the V.sub.BE (T) term that precludes the reference voltage from being truly temperature independent.
The reference voltage V.sub.REF can be expressed as a function of a physical constant, several device parameters, and, typically a single user driven parameter that determines the entire voltage-vs.-temperature characteristics of any conventional bandgap reference. The equation that expresses these relationships, which is derived by expansion of equation 1a, is the well-known Standard Model and is shown below: ##EQU3##
where V.sub.GO) is the so called "extrapolated bandgap voltage," .gamma. is a combined process, device, and operating current temperature coefficient variable, .alpha. and .eta. are device dependent parameters, and T.sub.0 is the user selectable parameter that defines the temperature at which V.sub.REF, reaches a peak value. Note that T.sub.0 can be altered by adjusting the scaling factor C in equation 1a. When T.sub.0 is chosen to occur at ambient temperature, i.e., T.sub.0 =T.sub.amb, which is the convention in the prior art, the resulting reference voltage V.sub.REF has a parabolic curvature as shown in FIG. 1. For more information relating to bandgap reference voltages and equation 2, see Gray & Meyer, "Analysis and Design of Analog Integrated Circuits," 289-96 (John Wiley & Sons, 2nd ed. 1984), which is incorporated herein by reference.
FIG. 1 is a graph showing the well-known temperature dependence of a reference voltage V.sub.REF generated by a bandgap reference voltage circuit without additional correction circuitry. As shown in FIG. 1, the non-linear curvature of reference voltage V.sub.REF is parabolic over the operational temperature range, i.e., between temperatures 220.degree. K and 400.degree. K. Voltage reference circuits are conventionally trimmed to minimize reference voltage errors by adjusting T.sub.0 so that the peak value of the reference voltage V.sub.REF occurs at ambient temperature T.sub.amb approximately 300.degree. K, as shown in FIG. 1. By assuring that the peak of voltage V.sub.REF is at ambient temperature T.sub.amb, the parabolic curvature of the reference voltage is approximately symmetrical about the peak value within the operational temperature range.
Conventionally, a pair of bipolar transistors forced to operate at a fixed non-unity current density ratio is used to generate a .DELTA.V.sub.BE (T) term, which is then scaled by C to produce the V.sub.TEMP term. The .DELTA.V.sub.BE (T) term is the difference between two base-emitter voltages, e.g., V.sub.BE1 and V.sub.BE2, of two diodes or transistors operating at a constant ratio between their collector-current densities, where collector-current densities are defined as the ratio between the collector current to the emitter size. Thus, .DELTA.V.sub.BE (T) can be expressed accordingly: ##EQU4##
where: ##EQU5##
and V.sub.BE1 and V.sub.BE2 are the respective base-emitter voltages of a two transistors or diodes, k is Boltzman's constant, T is the absolute temperature (.degree.K), q is the electronic charge, J.sub.1 and J.sub.2 are the respective current densities in two transistors or diodes, the ratio of which is intended to be fixed with regard to temperature.
As can be seen in equation 4, under the above conditions, the .DELTA.V.sub.BE (T) term is precisely linear with temperature. The temperature dependent thermometer voltage V.sub.TEMP is generated by combining the .DELTA.V.sub.BE (T) term with a scaling factor C as follows. ##EQU6##
The scaling factor C is typically used to amplify and adjust the value of the temperature dependent voltage V.sub.TEMP. FIG. 2 is a graph showing a temperature dependent voltage thermometer V.sub.TEMP generated by a circuit consisting of a pair of bipolar transistors operating at a fixed non-unity current density ratio to produce a .DELTA.V.sub.BE, which is then scaled by C.
It is necessary to distinguish here between the thermometer voltage V.sub.TEMP, which is produced in this example by the scaled .DELTA.V.sub.BE from two bipolor devices, and the .DELTA.V.sub.BE term in equation 1a, which is a component of V.sub.REF. Both are produced by the same mechanism in this example, i.e., by a pair of bipolar transistors, but they are not the same voltage. They may be from different pairs of bipolar devices in one embodiment of this invention, or may be produced from a single pair of bipolar devices, as an engineering choice, in another embodiment of this invention. In the following description, the term V.sub.TEMP is used to refer strictly to the thermometer voltage, which may or may not be reused to adjust the reference voltage V.sub.REF. Additionally, the thermometer voltage V.sub.TEMP may be produced by other means, and not only by the use of a pair of bipolor transistors, as discussed elsewhere in the present disclosure.
Combining equations 2 and 5 completes the right hand side of equation 1. Now, the thermometer transfer function F(T) for a conventional temperature sensor may be expressed as: ##EQU7##
Because a digital temperature sensor system divides the precisely linear voltage V.sub.TEMP by the reference voltage V.sub.REF, which has a parabolic curvature error, the thermometer transfer function F(T) of the digital temperature sensor system has a corresponding but inverted parabolic curvature error. FIG. 3 is a graph showing the temperature error in the thermometer transfer function F(T) generated by a conventional temperature sensor, which uses the ratio of V.sub.TEMP to V.sub.REF as stated in equation 1. As shown in FIG. 3, the resulting error of the temperature output signal is a non-linear curve that is the inverse of the parabolic reference voltage V.sub.REF, shown in FIG. 1.
Because of individual part errors, each digital temperature sensor will generate a different amount of error at ambient temperature T.sub.amb. Thus, individual digital temperature sensors are adjusted through conventional trimming such that there is a minimal error in the temperature output signal T.sub.OUT (C) at the ambient temperature T.sub.amb FIG. 4 is a graph showing temperature errors produced by eighteen non-trimmed digital temperature sensors. As can be observed in FIG. 4, when a digital temperature sensor has no error at ambient temperature T.sub.amb, e.g., when the digital temperature sensor requires no trimming or is trimmed to compensate for error at ambient temperature T.sub.amb, the familiar parabolic curve 20 is generated. The parabolic error curve 20 shown in FIG. 4 is similar to the error curve shown in FIG. 3 (with a different scale along the Y axis). Thus, despite trimming the digital temperature sensor to compensate for error at ambient temperature T.sub.amb, the digital temperature sensor will still produce an error at non-ambient temperatures.
FIG. 5 is a schematic diagram of a conventional monolithic digital temperature sensor 10. Temperature sensor 10 uses a bandgap reference voltage circuit 12 that generates a bandgap voltage reference V.sub.REF in accordance with equation 2 above. Temperature sensor also uses a thermometer circuit 14 to generate the linearly-temperature-dependent voltage V.sub.TEMP /C as described above in reference to equation 5. Thermometer circuit 14 is typically an additional pair of transistors or diodes operating in accordance with equation 5.
The bandgap reference voltage V.sub.REF is used to normalize the linearly-temperature-dependent voltage, V.sub.TEMP /C as produced by thermometer circuit 14. An analog to digital (A/D) converter 16 computes a digital fraction representing the ratio of V.sub.TEMP /C to V.sub.REF /C', where C' is nominally equal to C, to a resolution of N bits and produces a temperature output signal T.sub.OUT (K) in degrees Kelvin. The Kelvin equivalent of 0.degree. Centigrade, i.e., 273.15.degree. K, is subtracted from the temperature output signal T.sub.OUT (K) produced by A/D converter 16 at summing block 18. Thus, summing block 18 produces a temperature output signal T.sub.OUT (C) in degrees Centigrade. The temperature output signal T.sub.OUT (C) produced by digital temperature sensor 10 can therefore be expressed as follows. ##EQU8##
where T.sub.FS is the full scale digital output of the converter in Kelvin.
As discussed above, the temperature output signal T.sub.OUT (C) generated by digital temperature sensor 10 will include the parabolic error term, which is essentially the inverse of the characteristic downward facing parabolic error produced by the uncorrected bandgap reference voltage circuit 12. Conventional techniques, such as polynomial curvature correction, much greater than unity collector current temperature slopes, and non-linear resistor compensation, are typically used to mitigate the parabolic error term in the temperature output signal T.sub.OUT (C) by attempting to "flatten" the bandgap voltage reference V.sub.REF. Thus, errors in digital temperature sensors are conventionally corrected in the prior art by introducing additional complex temperature dependent circuitry that modifies the behavior of bandgap reference circuit 12. Unfortunately, such complex temperature dependent circuitry requires additional die area, consumes more power, increases both manufacturing and trimming complexities, and adversely affects reliability.
Thus, what is needed is a digital temperature sensor with improved error that does not require additional circuitry to correct the bandgap curvature voltage.